This invention relates generally to the regeneration of hydrocarbon conversion catalysts in the presence of a chlorine-containing species.
Although catalysts for the conversion of hydrocarbons have a tendency to deactivate, usually a catalyst""s activity may be restored by one of a number of processes that are known generally as regeneration processes. Regeneration processes are extensively used. What specific steps comprise a regeneration process depends in part on the reason for the deactivation. For example, if the catalyst deactivated because coke deposits accumulated on the catalyst, regeneration usually includes removing the coke by burning. If the catalyst deactivated because a catalytic metal such as platinum becomes agglomerated, regeneration usually includes redispersing the metal by contacting the catalyst with oxygen and chlorine. If the catalyst deactivated because a catalytic promoter such as chloride becomes depleted, regeneration usually includes replenishing the promoter by contacting the catalyst with a chlorine-containing species, which are referred to herein as chloro-species. Operating conditions and methods for these regeneration processes are well known. Regeneration processes can be carried out in situ, or the catalyst may be withdrawn from the vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for reactivation. Arrangements for continuously or semicontinuously withdrawing catalyst particles from a reaction zone and for reactivation in a regeneration zone are well known.
Many of these regeneration processes share the common feature of contacting the catalyst in the presence of one or more chloro-species that restore the activity of the catalyst for use in the reaction zone. These chloro-species may be chemically or physically sorbed on the catalyst as chloride or may remain dispersed in a stream that contacts the catalyst. In many regeneration processes, however, a flue gas stream containing the chloro-species is vented from the regeneration process. Several methods have been used for preventing contamination of the flue gas stream with the chloro-species and minimizing the release of the chloro-species in the flue gas stream from the regeneration process. Emissions of chloro-species, apart from the effect of the loss of chloride on the catalyst, pose environmental concern. The loss of chloride usually causes temporary deactivation that can be reversed by adding make-up chloride to the catalyst. The environmental concerns can be abated either by scrubbing the flue gas stream with an aqueous, basic solution that neutralizes the chloro-species or by adsorbing the chloro-species on an adsorbent. Scrubbing and adsorption are the two methods that are typically used when chloro-species are vented during regeneration of reforming catalysts and of catalysts for other hydrocarbon conversion processes, such as dehydrogenation, isomerization, alkylation, and transalkylation.
Although these two methodsxe2x80x94scrubbing and adsorptionxe2x80x94for decreasing the venting of chloro-species during catalyst regeneration are useful, they are also expensive to build and troublesome to operate. On the one hand, by introducing an aqueous solution into the process, scrubbing can actually increase the risk of downstream corrosion unless the alkalinity of the aqueous solution is carefully controlled. Moreover, because the aqueous solution must be replaced periodically, scrubbing gives rise to significant costs for supplying fresh solution and for disposing of the spent solution. On the other hand, although adsorption does not involve the introduction of an aqueous stream, the adsorbent also must be replaced periodically, and the cost of replacement of the adsorbent, including the cost of disposing of spent adsorbent, can far exceed the cost of replacement of the aqueous solution in scrubbing.
The problem of adsorbent replacement is compounded by water in the flue gas stream, and as a result traditional adsorbents are not economically viable for adsorbing chloro-species from flue gas streams. In order to be economically viable, an adsorbent, while removing a high proportion of the chloro-species from the flue gas stream, must adsorb typically from 7 to 8 percent of its weight in chloride. In order to adsorb that much chloride, the flue gas must have a low water content, typically less than 0.01 mol-% water. Water competes with chloro-species for adsorption sites on the adsorbent, and by occupying sites that would otherwise be occupied by chloro-species, water hinders the adsorption of chloride and hastens replacement of the adsorbent. Thus, if the flue gas has a high water content, the adsorbent adsorbs too much water and is incapable of adsorbing a viable amount of chloride. Because water is a common by-product of coke combustion as a result of the hydrogen-containing compounds typically found in coke, flue gas streams often do have a high water content, typically from 1 to 10 mol-%. As a consequence, unless the flue gas is dried an adsorbent will adsorb only one-third to one-half of the weight of chloride required for economic viability. This, in turn, doubles or triples the frequency of adsorbent replacement, thereby making traditional adsorbents uneconomical. Although in theory the adsorption of water can be mitigated by drying the flue gas stream prior to adsorbing the chloro-species, in fact a drier is costly as well as impractical because chloro-species such as hydrogen chloride tend to degrade most desiccants.
Thus, a process is sought for removing hydrogen chloride and other chloro-species from the flue gas streams of catalyst regeneration processes without the need for aqueous solutions, adsorbents, and desiccants.
It has been discovered that a spent catalyst having an alumina support entering a regeneration zone can sorb chlorine-containing species, which are referred to herein as chloro-species, from a flue gas stream from the regeneration zone, thereby dramatically reducing both the emissions of chloro-species from the regeneration zone as well as the make-up addition of chloro-species to the regeneration zone. In particular, this invention is applicable to regeneration zones that combust coke from coked, chloride-containing alumina particles, especially spent naphtha reforming catalysts and spent paraffin dehydrogenation catalysts. In order to take advantage of this property of these catalysts to sorb chloride from the flue gas, a chloride sorption step that can be readily integrated into existing regeneration processes without large capital expenditures or greatly increased complexity is provided. The sorption conditions are characterized by the substantial absence of carbon combustion.
In this invention, a sorption arrangement in combination with the regeneration section of a catalytic hydrocarbon conversion process retains chloro-species that would otherwise be released from the process. This invention uses a sorption zone to recycle the chloro-species to the regeneration zone. Unlike conventional adsorption methods of preventing release of chloro-species from catalytic regeneration zones by adsorbing chloro-species onto a separate adsorbent, this invention uses the catalyst entering the regeneration section to capture the escaping chloro-species and return chloride back to the regeneration section. This invention uses the catalyst entering the regeneration zone to keep the chloride in the regeneration zone, and to sustain the chloride level on the catalyst.
It has been discovered that, even though the catalyst entering the regeneration zone is like traditional sorbents in that it is capable of sorbing up to, say, only about from 2 to 3 percent of its weight in chloride from a water-containing regeneration flue gas stream, a process that uses the catalyst entering the regeneration zone to sorb chloro-species from the regeneration flue gas stream can nevertheless be viable. Accordingly, in one of its embodiments, this invention is a process in which spent catalyst which is about to be regenerated is not passed to the regeneration zone but instead is first passed to an sorption zone. In the sorption zone, the spent catalyst particles sorb chloro-species from the regeneration zone flue gas. Because the regeneration flue gas typically has a high water content, the spent catalyst sorbs up to, say, only about from 2 to 3 percent of its weight in chloride. The spent catalyst, having sorbed what chloride it can, is withdrawn from the sorption zone and is passed then to the regeneration zone. Whatever additional chloride the spent catalyst sorbed in the sorption zone is brought into the regeneration zone, thereby decreasing the need to add make-up chloride to the regeneration zone. Meanwhile, the sorption zone is replenished with a continual stream of spent catalyst, which is capable of being supplied to the sorption zone at a rate that is more than sufficient to compensate for the fact that the catalyst sorbs only up to about 2 to 3 percent of its weight in chloride. In short, in this invention the abundant quantity of available catalyst for sorption more than compensates for what persons skilled in the art would consider a small and uneconomical amount of chloride sorbed by the catalyst. The benefits of this invention for the regeneration process include not only a decrease in the emissions of chloro-species but also a decrease in the chloride make-up rate.
In regeneration processes as currently commercially practiced, the flue gas from a zone where coke is combusted from chlorided platinum alumina catalysts typically contains from 10 to 500 mol-ppm chlorine and from 500 to 10000 mol-ppm hydrogen chloride. By practicing this invention in which a high proportion of the chlorine and hydrogen chloride in the flue gas is sorbed on the coked catalyst prior to the combustion of the coke, the chlorine concentration in the flue gas may be reduced to from 1 to 10 mol-ppm and the hydrogen chloride concentration may be reduced to from 10 to 1000 mol-ppm. The method of this invention can be used to replace or supplement conventional means for removing chlorine and hydrogen chloride from flue gas streams, such as scrubbing or adsorption. In either case, this invention significantly lowers the substantial costs of building and operating the conventional means of chloro-species removal. In addition, this invention reduces significantly the requirements for adding make-up chloride to the regeneration process, because this invention returns to the regeneration process a large portion of the chlorine or hydrogen chloride that would otherwise be removed from the process by conventional means of chloro-species removal. Thus, the method of this invention can eliminate or drastically reduce the problems and costs associated with regeneration processes that emit a flue gas stream containing hydrogen chloride or chlorine.
This invention is applicable to numerous hydrocarbon conversion processes and to their associated catalyst regeneration zones. A basic requirement for using this invention is a zone that contains catalyst or particles and that operates in the presence of chloride which is carried out of the zone in the form of a chloro-species by a flue gas stream. Examples of chloro-species that are released from the zone and are susceptible to recovery by the method of this invention include Cl2 and HCl. Another basic requirement for using this invention is catalyst or particles that have sorption capacity for the chloro-species. This invention is not limited to any particular type of catalyst or particles; any catalysts or particles with the necessary capacity may be used. Preferably, the catalyst or particles will recover 50 wt-% and, more preferably, more than 90 wt-%, of the chloro-species in the effluent stream. The typical catalyst or particles for use in this invention comprise alumina, including alumina, activated aluminas, silica alumina, molecular sieves, and alumino-silicate clays such as kaolin, attapulgite, sepiolite, polygarskite, bentonite, and montmorillonite, particularly when the clays have not been washed by acid to remove substantial quantities of alumina. Reference is made to Zeolitic Molecular Sieves, by Donald W. Breck (John Wiley and Sons, 1974), which describes the use and selection of zeolite adsorbents and which is incorporated herein by reference.
The sorption and removal capacity of the catalyst or particles for the chloro-species must exist under a reasonable range of conditions. With respect to the removal capacity, the capability for chloride to be removed from the catalyst during regeneration is a necessary feature of the catalyst regeneration process to which the invention is applied. In other words, the conditions at which the regeneration process functions are conditions that are sufficient to remove chloride from the catalyst. As a practical matter, however, this requirement does not limit the scope of this invention in any significant way. With respect to the capacity of the catalyst or particles to sorb chloro-species, preferably the process conditions of the flue gas will complement the sorption requirements of the catalyst or the particles. In a surprising aspect of this invention, the sorption of chloro-species in the sorption zone is favored by a decrease rather than an increase in the pressure of the sorption zone. Although persons of ordinary skill in the art of sorption processes would have expected that a decrease in pressure would not have been beneficial for the sorption of gaseous chloro-species onto the catalyst or particles, it has been discovered that the opposite is true in the presence of water and at the temperatures of the sorption zone. It has been discovered that although a decrease in pressure causes the sorption of water from the flue gas onto the catalyst to decrease, the sorption of chloro-species does not decrease, even at the temperatures of the sorption zone. Therefore, a decrease in pressure selectively favors the sorption of chloro-species relative to that of water. Consequently, a preferred embodiment of this invention includes a sorption zone that operates at a pressure that is less than the pressure of the zone in which the chloride is removed from the catalyst or particles. A lower pressure may be compatible with some prior art hydrocarbon catalyst regeneration processes in which prior to the regeneration step the catalyst is employed for hydrocarbon conversion at a pressure that is lower than the pressure of the regeneration step. In these prior art processes, it is beneficial to perform the sorption of the chloro-species on the catalyst prior to increasing the pressure of the particles for the regeneration step. Therefore, this invention may be adaptable to existing and prior art processes and achieve substantial benefits with a minimum of utility requirements and additional capital expenses.
This invention is not limited to the recovery and recirculation of a single chloro-species from the regeneration zone, but may include arrangements for the recovery of two or more chloro-species. Preferably, the catalyst or particles in a single sorption zone will retain all of the chloro-species that are removed from the catalyst or particles into the flue gas and that are desired to be sorbed from the flue gas stream. Where necessary, multiple sorption zones with the catalyst or particles in each zone but operating at different sorption conditions may be used to recover the various chloro-species from the flue gas stream.
Thus, this invention uses sorption and removal steps in a catalyst regeneration process that results in the recovery and return of chloro-species to the process. The process is compatible with a wide variety of catalyst regeneration sections for hydrocarbon conversion processes. This compatibility can minimize utility costs by operating at conditions which are in harmony with the typical process conditions and existing process steps.
It is an object of this invention to improve processes for regenerating hydrocarbon conversion catalysts.
It is another object of this invention to make better use of chloro-species that are present during catalyst regeneration.
A further object of this invention is to decrease the costs that are incurred in venting chloro-species from catalyst regeneration processes.
Accordingly, in a broad embodiment, this invention is a method for decreasing the environmental release of chloro-species from a moving bed process for regenerating spent catalyst particles. A first portion of a recycle stream passes to a regeneration zone that contains chlorided catalyst particles. The regeneration zone operates at regeneration conditions to produce regenerated catalyst particles. The regeneration conditions not only at least partially regenerate at least a portion of the chlorided catalyst particles, but also remove at least a portion of the chloride from the chlorided catalyst particles. A flue stream comprising chloro-species is withdrawn from the regeneration zone. At least a portion of the flue stream passes to a sorption zone that contains spent catalyst particles. At least a portion of the chloro-species in the at least a portion of the flue stream is sorbed on the spent catalyst particles at sorption conditions, thereby producing the chlorided catalyst particles, which have an increased content of chloride relative to the spent catalyst particles. The sorption conditions are characterized by the substantial absence of carbon combustion. A recycle stream, which has a reduced concentration of chloro-species relative to the at least a portion of the flue stream, is withdrawn from said sorption zone. A second portion of the recycle stream is rejected from the process. At least periodically, catalyst particles are moved through the sorption zone and the regeneration zone by withdrawing regenerated catalyst particles from the regeneration zone, passing chlorided catalyst particles from the sorption zone to the regeneration zone, and adding spent catalyst particles to the sorption zone.
Other objects, embodiments and details of this invention are presented in the following detailed description of the invention.
U.S. Pat. No. 3,652,231 (Greenwood et al.) shows a regeneration apparatus in which a constant-width movable bed of catalyst is utilized. The ""231 patent also describes a continuous catalyst regeneration process which is used in conjunction with catalytic reforming of hydrocarbons. U.S. Pat. No. 3,647,680 (Greenwood et al.) and U.S. Pat. No. 3,692,496 (Greenwood et al.) also deal with regeneration of reforming catalyst. The teachings of patents (""231, ""680, and ""496) are hereby incorporated in full into this patent application.
U.S. Pat. No. 5,336,834 (Zarchy et al.) discloses an adsorption zone in combination with a catalytic hydrocarbon conversion process that keeps chlorine-containing compounds in the reaction zone and prevents contamination of product streams with chlorine-containing compounds.
U.S. Pat. No. 4,218,338 (Huin et al.) discloses a process for regenerating a hydrocarbon conversion catalyst wherein the gas discharged from the regeneration zone is cooled, subjected to double washing, dried, compressed, heated, and reused in the regeneration zone.
Temperature control and chloride management during regeneration of fixed beds of catalyst are described in the article entitled xe2x80x9cCat Reforming With In-Place Regeneration,xe2x80x9d written by W. H. Decker et al., and published in the Jul. 4, 1955, issue of The Oil and Gas Journal beginning at page 80, and in the discussion at pages 355-397 in the book entitled Progress in Catalyst Deactivation, edited by J. L. Figueiredo, and published by Martinus Nijhoff Publishers in Boston, Mass. in 1982.